1. Field of Invention
The present invention is directed to anterior prosthetic intervertebral devices which may be inserted in the vertebral disc spaces resulting from the removal of diseased or damaged intervertebral discs. The present invention is also related to prosthetic intervertebral devices for insertion in the space resulting between non-contiguous vertebrae following a corpectomy. In particular, the present inventive intervertebral devices have specific, fixed shapes designed to accommodate the normal morphological anatomy of anterior vertebral endplates, particularly in the thoracic and lumbar regions of the spine, for better stability and fit. The present invention is further related to a method of determining the specific morphology of the surfaces of vertebral bodies, more particularly anterior vertebral endplates.
2. Description of the Related Art
Devices for Intervertebral Disc Space Repair:
In surgery, there are frequently indications for total or near total removal of an intervertebral disc from an anterior approach. On some occasions, a corpectomy is required wherein the entire vertebral body itself is removed because of fracture, tumor, or deformity. The subsequent space created by these procedures which corresponds to the space vacated by the disc (or by two discs and the intervening vertebral body resulting after a corpectomy) needs to be reconstructed in surgery.
One of the most well-established methods of intervertebral space reconstruction involves the careful placement of an autograft (i.e. bone graft) between the two vertebral endplates (i.e. the superior endplate of one vertebra and the inferior endplate of a second vertebra). The autograft will bear weight across the surgical defect and ultimately initiate bony union or healing between the graft and the adjacent vertebrae. These autografts, which can be obtained from the fibula or the pelvis, are excised from the patient undergoing surgery and then shaped by the surgeon to fit the proper intervertebral space.
Another method of intervertebral space reconstruction is the use of allografts. Allografts are bones obtained from another human individual (most are usually harvested from voluntary donors after death). These allografts can be prepared in numerous ways, with the process including aggressive washing and radiation sterilization, and can be produced in a variety of sizes and shapes. Allograft sections of human femur are a popular choice and can be purchased with flat upper and lower surfaces in various heights. These allograft femur segments are shaped and sometimes shortened by the surgeon at the time of surgery, and then placed into the spaces created by the surgical removal of a disc or vertebral body. As with the autografts, these allografts are intended to heal the adjacent vertebral endplates together.
There are also available intervertebral devices designed to augment the weight bearing capacity of the applied graft as well as allow the use of morselized or fragmented non-structural grafts. These devices include shaped titanium cages and graphite (carbon fiber) boxes, both of which are manufactured in a variety of sizes and lengths. The titanium cage is manufactured with a flat top and bottom. The graphite box is also currently manufactured with parallel flat surfaces on the top and bottom. Both devices are intended to bear weight across a corpectomy interval or intervertebral disc space interval and allow the packing of cancellous autografts or allografts into the space.
A more elaborate device intended to accomplish a similar purpose as the titanium cage and graphite box is the AMSET Rezaian Spinal Fixator. This mechanical device is designed to be placed into defects to allow a mechanical distractive force to be applied through the created defect. This device also has a flat surface on top and bottom which is augmented by conical teeth which engage the superior and inferior vertebral endplates.
Another device is Spine-Tech, Inc.'s BAK Spinal Stabilization System. This device is a threaded cylinder intended to be placed across a disc space to obtain screw fixation in the superior endplate of one vertebra and the inferior endplate of a second vertebra.
Kaneda, K., et al. (Spine, 17: S295-S303 (1992) report the use of ceramic vertebral prostheses, more specifically the use of these ceramic implants along with the Kaneda device for the treatment of osteoporotic posttraumatic vertebral collapse. These implants are generally rectangular in shape and have substantially flat upper and lower surfaces. The researchers report on page S296 that the vertebral body surfaces should be shaved to touch firmly the ceramic implant.
A very different approach to intervertebral space reconstruction is total disc replacement. One total disc replacement design is the Steffee design manufactured by AcroMed. This device consists of a deformable plastic insert attached to a flat metal surface above and below. These flat metal surfaces have conical teeth, similar to those of the Rezaian Spinal Fixator, that are intended to engage the vertebral endplate. Another deformable plastic disc analog is that designed by Dr. Casey Lee of New Jersey Medical School which is contoured with arbitrarily selected curved surfaces above and below the implant.
Another intervertebral disc utilizes the Kostuik disc design. This disc also has a flat upper and lower endplate; however, instead of a deformable plastic disc analog, this implant uses metal mechanical springs.
U.S. Pat. No. 5,171,278 to Pisharodi is directed to an expandable artificial disc prosthesis. More specifically, it is directed to cylindrical and rectangular disc implants which are expandable in the middle to contact the vertebral bodies.
None of the foregoing devices are designed to accommodate the defined anatomical contours of the vertebral endplate. Consequently, these devices contact only a minimal number of points on the surfaces of the vertebral endplates. Such an uneven distribution of stress exerted by the adjacent vertebrae upon the devices further results in an increase risk of subsidence and collapse of the device.
U.S. Pat. No. 5,123,926 to Pisharodi is directed to a spring-loaded, middle expandable total disc prosthesis. The disc is comprised of an elastic bag having a plurality of spikes which extend upward from the superior surface, and downward from the inferior surface of the disc. The disc may be expanded to fill the disc space by injecting a liquid or gas substance into the disc. Such expansion of the disc results in an increase number of contact points between the disc and the surface of the vertebral endplates. However, the elastic disc is much more susceptible to wear and ultimately collapsing once implanted in the human body due in part to the nature of the elastic material (rubber, silicone rubber, and plastic, for example) required for disc expansion. Moreover, the design of the disc precludes the incorporation of osteoinductive materials, such as bone growth factors, for example, which assist in bone fusion.
The goal in practically every case of intervertebral disc space reconstruction is to achieve bony fusion between the vertebral endplates and the intervertebral device. Unfortunately, bone grafts do not always heal reliably, with some studies reporting failure rates (i.e. failure of adequate bone fusion) ranging from 10% to as high as 40%. Without complete bone fusion of the vertebral endplates with the intervertebral device or graft, the vertebrae adjacent to device or graft is less stable, often necessitating further surgery. Consequently, attempts have been made to facilitate bone growth. One technique is to apply electrical stimulation to the graft, as accomplished by several devices manufactured by the E.B.I. Company. The application of electrical stimulation to the bone is theorized to promote bone growth into the device.
It is therefore desirable to have an intervertebral device for use in intervertebral disc space reconstruction resulting from the removal of a single disc, or a total corpectomy, that:
1) has defined contours or shapes that are designed to accommodate the normal and predictable morphological anatomy of the vertebral endplates, resulting in significantly better stress distribution along the endplate; PA1 2) is formed of a durable and physiological compatible material that will better endure the forces exerted upon it by the adjacent vertebral bodies; and/or PA1 3) is formed of, or designed to, accommodate an osteoinductive material such as bone growth factors to facilitate bone fusion into the intervertebral device faster and more reliably for better repair of the disc space.
Morphology of vertebral bodies:
The increase in popularity of lumbar interbody fusion and the design of many interbody implants has created the need for a more quantitative anatomical description of the vertebral column. In particular, recent investigations of vertebral mechanics have shown that the vertebral endplate plays an important role in supporting stresses passed through the intervertebral disc. This leads to the suggestion that endplate resection during surgery may compromise the ultimate strength and/or stability of the surgical construct.
The literature contains many studies of the anatomy of the spinal column. J. L. Berry, et al "A Morphometric Study of Human Lumbar and Selected Thoracic Vertebrae," Spine, 12:362-367 (1987)! measured twenty-seven dimensions in the thoracic and lumbar vertebrae using dried spinal columns from thirty skeletons. In addition to overall dimensions, they record the angle between inferior and superior endplates. M. Nissan and I. Gilad "The Cervical and Lumbar Vertebrae--An Anthropometric Model," Eng. Med., 13(3):111-114 (1984)! performed a study of vertebral dimensions which also described the overall angle between endplates on each vertebra. P. V. Scoles, et al. "Vertebral Body and Posterior Element Morphology--The Normal Spine in Middle Life," Spine, 13:1082-1086 (1988)! conducted a study on vertebral body morphology which was focused primarily on the posterior elements.
M. M. Panjabi et al "Thoracic Human Vertebrae--Quantitative Three-Dimensional Anatomy," Spine: 16:888-901 (1991)! conducted an extensive study on the three-dimensional disc anatomy of thoracic and lumbar vertebrae. These researchers devised their measurements into linear parameters, surface and cross-sectional area parameters, and angular parameters. Measurements of the vertebral body were limited to endplate width, endplate area, and endplate inclination (i.e. the angle between the best-fit planes for each endplate).
For purposes of developing intervertebral implants designed to better fit the specific contours of the surfaces of adjacent vertebral bodies, particularly vertebral endplates in the thoracic and lumbar spinal regions, it is desirable to have a method for quantitatively determining the three-dimensional morphology of these vertebral surfaces. Such a method would be useful in designing a series of intervertebral devices and implants having defined surface shapes which compliment or accommodate the defined morphology of the adjacent vertebral surfaces. Additionally, such a method could be used to measure individual vertebral surfaces of an individual patient for purposes of customizing an intervertebral endplate to accommodate the patient's individual anatomy.